The degree to which preventive detention furthers its community safety purpose depends entirely upon the capacity to predict who will commit a crime over a specified period of time. Jurisdictions typically rely on three factors for determining pretrial dangerousness: prior criminal record, seriousness of the current offense and judicial discretion. However, there is little empirical evidence that these charge-related bases for detention are good indicators of criminality during the pretrial period. As applied, preventive detention reflects some combination of actuarial and clinical predictions involving judicial experience and normative expectations. While predictions of a broad range of pretrial crimes are efficient, the prediction of dangerousness is unreliable. When narrowed to violent crimes, the decision standard that guides preventive detention statutes in many state predictions is inefficient and the performance is unacceptable. In light of the great cost to defendants in terms of case outcomes and sanctions, and the marginal gains to society in crimes averted, preventive detention appears to be unjustified. Notes, tables